1. Field of the Invention
The present invention relates to viewfinder optical systems suited to be used in lens shutter cameras or video cameras and, more particularly, to a viewfinder optical system which utilizes a surface of rotational asymmetry to minimize the size of the entire optical system, while still permitting a non-inverted erecting viewfinder image of good quality to be observed.
2. Description of Related Art
Heretofore, for viewfinder systems adapted for an photographic cameras or video cameras, there have been proposed various viewfinder optical systems of the real image type in which a viewfinder image that is a real image formed by an objective lens on a primary image forming plane is observed through an eyepiece lens. Such a viewfinder optical system of the real image type is easier than a viewfinder optical system of the virtual image type in obtaining a compact form of the entire optical system, and is, therefore, in recent years, used in a great number of zoom-lens-equipped cameras.
As a viewfinder optical system of the real image type, mention may be made of, for example, Japanese Laid-Open Utility Model Application No. Sho 58-62337, in which an object image (viewfinder image) formed on the primary image forming plane by the objective lens is converted into a non-inverted erecting viewfinder image by an image inverting means such as a porro-prism, and the non-inverted erecting viewfinder image is observed through the eyepiece lens. Further, in Japanese Laid-Open Utility Model Application No. Hei 1-160427, there has been proposed a viewfinder optical system of the real image type in which a roof-type reflecting member, two mirrors, etc., is disposed in the optical path for the purpose of converting a viewfinder image into a non-inverted erecting image.
In the viewfinder optical system of the real image type using an image inverting means such as a porro-prism for obtaining a non-inverted erecting image, the objective lens for forming an object image on the primary image forming plane and the image inverting means are clearly distinguished from each other in the optical function. In other words, the conventional image inverting means makes no contribution to an optical power in forming the object image on the primary image forming plane.
For this reason, in the case of a viewfinder optical system in which a part of the image inverting means (for example, a prism member) is disposed in the space between the objective lens and the primary image forming plane, the back focal distance of the objective lens must be made long. For this purpose, it is necessary to employ the retro focus type in making up the objective lens, and the degree of retro focus has to be strengthened depending on the back focal distance. Therefore, the objective lens tends to increase in size.
Further, in the case of a viewfinder optical system in which a part of the image inverting means is disposed in the space between the objective lens and the primary image forming plane and a total-reflection surface for bending the optical path is disposed on the object side of the primary image forming plane, it is necessary to form the objective lens into an exit-side telecentric system so as to enable the total-reflection surface to effectively totally reflect the entirety of a light flux. As a result, the objective lens tends to increase in size.
Further, in the case of a viewfinder optical system of the secondary image forming type in which a viewfinder image formed on the primary image forming plane by the objective lens is relayed to a secondary image forming plane by a relay lens system (relay system) and the viewfinder image formed on the secondary image forming plane is observed through the eyepiece lens, an increased number of lenses are required for re-forming the viewfinder image. Therefore, the total length of the entire optical system becomes longer. Moreover, to correct chromatic aberrations, even larger a number of lenses becomes required.